One of the most critical steps in the wet-processing of semiconductor wafers or similar flat media, (such as photomasks, flat panel displays, hard disk media, CD glass, etc.) is the drying. Any rinsing fluid that remains on the surface of a semiconductor wafer has potential for depositing contaminants that may cause defects in the end product electronic device. In practice, deionized ("DI") water is used most frequently used as the rinsing fluid. Like most other fluid after rinsing, DI water will cling to wafer surfaces in sheets or droplets, due to surface tension. Ideally, the drying process should operate quickly and cost effectively to remove fluid and leave absolutely no contaminants on the wafer surface, while presenting no environmental or safety risks.
Various technologies have been used in the past to dry wafers and reduce the level of contaminants left on the wafer surface after drying. Whether a particular drying technology is appropriate for drying a wafer often depends on the affinity of the wafer surface for water in other words, whether a wafer surface is hydrophilic or hydrophobic. It is especially difficult to completely dry patterned wafers which have both hydrophilic and hydrophobic regions present, due to the varying droplet characteristics--large isolated droplets in the hydrophobic regions and a water film covering the hydrophilic regions.
Some drying technologies that have been used in the past include the following:
A. Spin-Rinse Dryers. These dryers operate on two fundamental mechanisms. First, the wafer is set into spinning motion to remove bulk liquid from the wafer surface by centrifugal force. Following removal of the bulk liquid, some liquid remains on the wafer surface because surface tension between the substrate and the residual liquid is greater than the level of centrifugal force that can be reasonably applied to the wafer especially near the center of the wafer. A second mechanism, evaporation, is then used to complete the drying process. The evaporation rate is commonly increased by maintaining a relatively high rotational velocity on the wafer, thus improving convection. Heated nitrogen gas is typically injected into the process chamber to further increase evaporative drying.
This drying technology is limited, however, by the following factors: (1) With hydrophobic surfaces, water drops can tend to become isolated on the water surface and are difficult to remove, and contaminants entrained in such droplets are deposited on the wafer surface. (2) Very high spin velocities, which improve drying, must be avoided due to the mechanical stress limits of the wafer and the tendency of high rotation speeds to generate turbulence which can cause contaminants to deposit on the wafer surface. This limits manufacturing throughput rates.
B. Isopropyl Alcohol (IPA) Vapor Dryers. These dryers operate by immersing wafers wetted with DI water into a heated environment saturated with IPA vapor. Liquid IPA has a significantly lower surface tension than water. In the typical IPA vapor dryer, the vapor chamber itself is heated to maintain an interior wall temperature of around 80-120 degrees C. As IPA starts to condense on the wafer surface, water on the surface is displaced by IPA. The wafer is maintained in the vapor zone for about 5 minutes, which allows the wafer to become quite warm. When the water has been displaced by IPA, the wafer is then withdrawn through a cool zone (having a series of heat exchange coils operating at about 50 degrees C. or less) which completes the condensation of the alcohol and causes it to flow off of the wafer surface. If any water dries in place on the wafer, however, it will leave a water mark in the form of a spot or streak which is detectable by a particle counter.
This drying technology is limited by the following factors: (1) It involves the inherent hazard of causing IPA, a flammable liquid, to be boiled at a temperature well in excess of its flash point. (2) It requires the consumption of IPA at relatively high rate. (3) It creates relatively high fugitive organic vapor emissions.
C. Leenaars et al., U.S. Pat. No. 5,271,774, discloses a device for removing liquids from the surface of a wafer using rotary motion in conjunction with an unsaturated solvent vapor contacting the wafer. The vapor is delivered to the wafer in an unsaturated state to prevent condensation on the wafer surface so as to avoid an additional drying treatment. The vapor is miscible with the liquid film on the wafer, resulting in a mixed-liquid film on the wafer having a surface tension lower than the original liquid film. This mixed-liquid film is then removed from the wafer surface by centrifugal forces generated by rotation of the wafer as the vapor continues to be passed over the wafer surface.
This drying technology is limited by the following factors: (1) Mixing between the unsaturated vapor and the liquid film on the wafer surface occurs slowly. This limits the manufacturing throughput rates. (2) It is not useful for drying more than one surface of a wafer at a time.
In light of the limitations inherent to these and other drying processes, it is an object of the present invention to provide a novel process for drying semiconductor wafers or similar items quickly and safely while leaving minimal levels of particle contaminants or chemical residue.
It is a further object to provide a novel process for drying patterned semiconductor wafers or similar items having both hydrophobic and hydrophilic regions.
It is yet a further object of the invention to accomplish such drying while reducing the hazards and emissions associated with drying wafers or similar items using volatile chemicals.